It is always best to test more at the beginning because it helps you gain experience and confidence in your test process.
The XL additional kit is for testing Chlorine only as most people tend to use that the most - almost daily during the swim season. The TF-100 kit comes with sufficient reagents to do a lot of tests. See the comparison that shows the estimated number of tests for each in this link -
Test Kits Compared - Trouble Free Pool | Trouble Free Pool
You may test chlorine more often than that of Calcium or Alkaline so you won't run out of everything at the same time. Also, some tests require only 2 or 3 drops from 1 reagent and 10 or more from others. After 6 months you will get a feel of what tests you need to do more often and how long each set of reagents will last. They sell individual reagents or a group of them so you can always make adjustments. Don't overbuy on reagents at the start because you do not want them to expire - all are good for 12 months and some even longer. However, I would buy the XL kit because that is very useful and used most often.
I also support buying the speed stirrer because it will make testing more consistent. Suggest you buy some extra stirrer beans (only 1 comes with it). Mine got mixed with my paper towel and into the trash by accident.
So both the speed stirrer and the XL option would be good addons (with the discount) with purchase of the TF-100 kit.
It is noted that you have a SWCG so you should add the K-1766 kit to your initial purchase as well unless you already have that kit.
Hope this helps.